My chapter evaluating academic research about paywalls and news payment systems will be published in Encyclopedia of Journalism Studies later on this year. The chapter also updates payment systems related to Facebook, and investigates micro payments and membership and donor models. The book is published by Oxford University Press, and it is edited by Henrik Örnebring.
The chapter, which is peer-reviewed, will also be published online. You can find it here once it is released:
Myllylahti, Merja. “Paywalls and Payment Systems.” In Encyclopedia of Journalism Studies. Oxford University Press. doi:10.1093/acrefore/9780190228613.013.855